The present invention refers to an actuator of the thermal type, and a to a method for closing and sealing the body of an actuator of the thermal type.
Actuators of this kind are known and used in various fields, among which the field of household appliances, air conditioning, hygienic-sanitary installations, motor-vehicles.
Such actuators usually consist at least of a thermosensitive element, which includes a body made of a thermal conductive material (such as metal), which is submitted to a heat source.
In the above body a chamber is provided, for containing a material (such as a wax) being expandable as a function of the temperature and, at least partially, a shaft or pusher is provided; to this purpose, the body has an opening defined on one of its longitudinal ends, which is sealed by a suitable device.
The consequence of a heat transmission to the body of the thermosensitive element is an expansion of the expandable material; such an expansion determines a linear displacement of the pusher to the outside of the body, until a determined position is reached, generally established by a mechanical limit stop, which can be defined as a stable work position. When the heat transmission to the body of the thermosensitive element decreases or is interrupted, the expandable material shrinks and let the pusher gradually return to its initial rest position, also with the aid of an elastic recall element, such as a spring.
In some solutions, the heat source for the body of the thermosensitive element may be an electric heater, such as a resistor with a positive temperature coefficient (PTC) supplied through two terminals or contacts; this solution reflects the instance of the actuator devices used in the automotive or household appliance fields.
According to other solutions, the actuation of the thermosensitive element is determined by the temperature of the environment where it operates: This is the case of mixers connected to water supply pipes of hygienic-sanitary installations in residential building construction, where the body of the thermosensitive element is subject to the temperature of the water flow to be regulated.
Thermosensitive elements as mentioned above can be manufactured in a simple and cheap way; their significant advantages are constituted by the considerable power they can develop, having regards their limited dimensions, and the low operation noise.
In order to seal the chamber containing the expandable material and obtain the required sealing on the pusher, a suitable tight closure device is commonly provided, which are housed in one same seat in correspondence of the above longitudinal opening of the body; the device usually includes a bushing in elastic material, typically Teflon.RTM., and two metal washers, i.e. an upper one and a lower one, which "pack" or "sandwich" the bushing. The above bushing has to be inserted on the pusher with interference, in order to warrant the required sealing, and this fact causes significant friction effects on the pusher itself.
The problem is made more critical by the fact that the bushing can be compressed or squeezed between the two metal washers, when the end of the body where the above opening is delimited is mechanically riveted, i.e. folded or "choked" on the upper washer, to obtain the tight closure of the chamber containing the expandable material; this may cause too high a compression of the bushing on the pusher, which makes the above problem more crucial.
It should also be pointed out that the Teflon.RTM. bushing tends to expand its volume when the body of the thermosensitive element is heated during operation; since the bushing is compressed in a single seat, its expansion causes further friction effects on the pusher.
Moreover, such an expansion has the effect of causing microextrusions of the material of the bushing: in other words, small portions of the material tend to enter the very small side passages existing between the central holes of the metal washers and the pusher passing through them; such leakages of the bushing lead to an early degradation of the sealing and, consequently, of the thermosensitive element.
The above drawbacks are such to generate substantial malfunctions of the thermosensitive elements, in most of their common applications; the use of thermosensitive elements so conceived can be especially critical in those instances where a high sensitivity of the device to the thermal changes induced to the body of the same and a considerable operation speed are required.
From U.S. Pat. No. 5,572,869 an actuator of the thermal type is described, including a thermosensitive element, wherein a sealing device is provided, in the form of a bushing and a seal ring; a schematic section of such a thermosensitive device is shown schematically in FIG. 1.
This known thermosensitive device, indicated as a whole with 100, has a body made of metal material 101, within which a chamber 102 is defined, containing a wax 103 or similar material which expands with the increase of temperature.
Reference 104 indicates a sealing ring (O-Ring), while 105 and 106 indicate a bushing in Teflon.RTM. and a brass washer respectively; the bushing 105 has a restrained size compared to the dimensions used in the above state of the art.
Reference 107 indicates a stainless steel shaft, a portion of which extending outside the body 101, through the holes of the center of the ring 104, of the bushing 105 and of the washer 106.
For the assembly purposes of the element 100, the chamber 102 is filled with a preset amount of wax 103 and then the shaft 107 is inserted into it, making sure that both elements 104 and 105-106 preliminarly associated with the shaft reach their respective work seats; in particular, following the assembly, the ring 104 is positioned in a housing 108, defined by a cylindrical widening of the chamber 102, while the bushing 105, which is inserted with interference on the shaft 107, and the washer 106, are positioned one on top of the other in a seat 109, being also defined by a cylindrical widening of the chamber 102, obtained above the housing 108 and having a larger diameter than the chamber.
Now, the upper end of the body 101, defining the opening through which the above elements can be inserted in the chamber 102, is mechanically riveted, i.e. folded over the washer 106, so as to obtain a tight closure of the chamber 102, locking both the bushing 105 and the seal ring 104 in the seat 109 and the housing 108, respectively.
The thermosensitive element described in U.S. Pat. No. 5,572,869 has some drawbacks, being due to the type of assembly of its components.
A first problem is due to the fact that a proper housing seat is not provided for the seal ring 104, for the following reasons.
Referring to the device described in U.S. Pat. No. 5,572,869, the seal ring 104 is in direct contact with the bushing 105, whose function--according to the current technique--should have the function to prevent portions of the ring 104 from entering the side spaces between the central hole of the washers 106 and the shaft 107 passing through it, towards the outside of the body 101 during the shaft movements 107 and due to the wax thrust.
However, according to the above prior art document, the working position of the bushing 105 is not at all certain, as it will become apparent later, and there is no suitable element provided underneath the ring 104 to delimit a precise seat.
As it can be seen in FIG. 1, in fact, the ring 104 is resting underneath on a shoulder 108A defined by the lower end of the housing 108, so that there is no well delimited basement on which the ring can possibly adapt itself: as a result, there is a situation where the ring is only performing a radial sealing on the shaft, as provided by the usual technique: however, in the specific application, such a contribution to tightness seems to be a moderate and irregular one.
Considering, in fact, that the shaft 107 is subject to movements in both directions, it is obvious that after a certain number of actuations the ring 104 may also be subject to settling movements downwards (it is pointed out that actuators as mentioned above should perform some tens of thousand actuations).
Another problem arising from the arrangement shown in FIG. 1 is that, according to U.S. Pat. No. 5,572,869, the bushing 105 is placed In the same seat 109 directly underneath the washer 106.
This determines the risk for the bushing 105 to be pushed downwards, by the washer 106, on the lower shoulder 109A of the seat 109, during the riveting operation of the body 101. As a result, the bushing runs the risk of being compressed on the shaft and even deformed on its lower part, i.e. a much higher deformation than that being typically caused during the usual operation of the device: in fact, the bushing 105 rests directly over a small sized shoulder (109A) and is subject to settlements, under the thrust of the washer 106, which turn to device malfunctions with time.
The thrust downwards of the bushing 105 and the resulting deformation then contribute to an excessive compression on the seal ring 104, which may become irregularly distorted since it has no suitable resting base.
Moreover, under the thrust of the bushing 105, the ring 104 may undergo settlements on the edges of the shoulder 108A, and the settlements may even change the volume of the chamber 102 containing the wax 103, so cause an undesired change in the feedback of the thermosensitive element, i.e. in its operation; to this purpose, it should be noticed that just a few tenths of millimeters of displacement of the ring 104 downwards, and therefore a minimum change of volume of the chamber 102, can lead to a high dangerous change of the device thrust power.
A further problem of the actuator described in U.S. Pat. No. 5,572,869, and somehow associated with the previous drawbacks, is due to the fact that no suitable guiding device towards the more internal portion of the body 101 is provided for the shaft 107.
In particular, small oscillations or operation clearances of the shaft 107 may cause a widening of the central hole of the bushing 105, thus further reducing its performance and leading to a radial slackening of the radial seal of the ring 104. This is due to the fact that the shaft 107 is practically maintained in its position by the washer 106 in stiff material alone; underneath this washer, in fact, the shaft 107 is only constrained by a ring 104, made of elastic material for its own nature, i.e. yielding easily, and by a bushing 105 also possibly subject to distortion phenomena during the operation of the device and/or the riveting operation of the body 101.